1. Field of the Invention
The present invention relates to an optical film with a pattern on the surface thereof, wherein the pattern is formed by applying a photosensitive composition to the surface of a transparent film and then irradiating the composition.
2. Description of the Background Art
Glass has excellent optical characteristics such as a high transmittance in the visible light range. It is free from problems associated with retardation. In addition, glass has superior heat resistance such as a high deflection temperature under load and is less susceptible to dimensional changes due to water absorption. Because of these advantages, glass has been used widely for various optical applications. A technique of forming a pattern on the surface of a glass plate by coating its surface with a photosensitive composition and then irradiating the composition is well known in the art, and is applied to photoprocessing of optical disc stampers and the manufacture of optical discs by the 2P method.
However, because glass is easily broken and is heavy, using glass as a substrate or a base occasionally causes problems in the final product such as portable equipment. In addition, because glass is not flexible and difficult to process, there are limitations to the shape of the final products. Furthermore, because glass can be rolled up only with difficulty, its handling is restricted and the productivity of final products is poor.
Because of these reasons, films or sheets made of a transparent resin, such as polyethylene terephthalate (PET), polymethyl methacrylate (PMMA) or polycarbonate (PC), have been used instead of glass as a substrate or a base on many occasions.
For example, there is a case where a film with a pattern on its surface is used in liquid crystal display devices for the purpose of promoting light diffusion and brightness. When a PET film is used for this application, the resulting images are darker and the image quality is poorer than in the case where glass is used. This is due to the low light transmittance of PET. In the case of PMMA, the conditions under which the final products are used are restricted because of poor heat resistance and large water absorption which leads to dimensional changes in the products. PC has large retardation and its heat resistance is not necessarily sufficient. Because of these reasons, when the final products are used in automobiles, for example, deformation by heat must be taken into account. This imposes limitations on the application of PC.
Further, in the case where a film provided with a holographic pattern on its surface is used for holography, PET or PC produces errors or distortions in the resulting holographic image due to their high retardation. As a result, it is difficult to precisely reproduce the original image. PMMA also has the problem of limitations to the conditions under which the final products are used because of the poor heat resistance and the large dimensional changes due to the large water absorption. A method of providing a holographic pattern by irradiating a photosensitive composition is frequently used because of the advantage of high productivity. PET or PC has a higher refractive index than the glass used for this application. Because of this, when used together with a gelatin or acrylic photosensitive composition which is commonly used with glass bases, phenomena such as reflections occur at the interface of the film and the photosensitive composition due to the difference in the refractive indices. This may result in an impaired holographic image. PMMA imposes limitations on the method of irradiation of a photosensitive composition because of its problems in heat resistance and deflection by water absorption. That is, if the photosensitive composition is irradiated through a patterned mask, developed, and washed, the drying temperature after the development and washing is restricted, which causes the productivity to be lowered. Another problem is restriction on the use of water in the development and washing processes.
The present invention has been developed in view of this situation and has an object of providing an optical film which breaks with difficulty, is light in weight, flexible, and easily processed. Owing to these characteristics, the optical film affords an extended freedom with respect to the shape of the final product and provides high productivity. Due to the use of a base film with the same optical characteristics as glass, the optical film can be widely used for various optical applications in which glass has normally been used.